Problem: Which of these numbers is prime? ${15,\ 18,\ 59,\ 75,\ 93}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 18 are 1, 2, 3, 6, 9, and 18. The factors of 59 are 1 and 59. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 93 are 1, 3, 31, and 93. Thus, 59 is a prime number.